We have observed that the antibiotic resistance pattern and plasmid composition of staphylococcal communities change appreciably in response to penicillin therapy of the host. Not all of the species respond similarly. However, a large proportion of the species and strains acquire multiple antibiotic resistance which is correlated with the acquisition of plasmids of similar size within a community, suggesting the possibility of plasmid transfer. These results prompted us to propose an investigation of the adaptation processes used by natural Staphylococcus communities in response to antibiotic pressure. Staphylococci are of growing medical importance, as it is becoming clear that modern practices leading to the compromise of patients (e.g., implantation of medical devices and immunosuppressive therapy) have greatly enhanced the risk of infection by coagulase-negative Staphylococcus species. The first aim of this research proposal is to determine the effects of various antibiotic therapy on the structure of staphylococcal communities living on the skin of the scalp, forehead, external auditory meatus, axilla, inguinal and perineal area, and interdigital spaces of the foot and on mucous membranes of the anterior nares and nasopharynx. We will be examining communities living on persons receiving benzylpenicillin and/or phenoxymethylpenicillin, methicillin and/or nafcillin, cephalothin and/or cefamandole, gentamicin, vancomycin, erythromycin, and tetracycline therapy. Strains will be examined for a variety of physiological and biochemical parameters, antibiotic susceptibility, and plasmid compostion, to determine their identity and nature of responses to the various types of antibiotic therapy. The second aim will focus on the origin of antibiotic resistant populations resulting from antibiotic thereapy. We will attempt to determine if resitant clones were merely selected from the original population, acquired from other sources, or resulted from genetic exchange (e.g. plasmid transfer) between members of the community. Genetic exchange will be assessed by conducting molecular analyses of plasmids. This will include restriction endonuclease analyses, colony hybridization, DNA-DNA filter hybridization, and heteroduplex analysis. Later in the investigation, as a third aim, we will attempt to reconstruct plasmid transfer in chemostat and microcosm communities to define the molecular processes.